Governments are increasingly regulating the exhaust emissions of vehicles. In particular, vehicles powered by diesel engines must meet more and more stringent regulations in the next several years. Cellular ceramic filters have been recognized as being useful in trapping particulates from exhaust emissions. As the filters become clogged, however, they must be regenerated or an unacceptable back pressure develops. A number of positive regeneration systems have been proposed which use variously, for example, a fuel fed burner (see U.S. Pat. No. 4,167,852), an electric heater (see U.S. Pat. Nos. 4,270,936; 4,276,066; 4,319,896), and detuning, techniques (see U.S. Pat. Nos. 4,211,075 and 3,499,269).
The present invention is directed to a regeneration system of a type which uses an electric heater to heat the inlet end of a ceramic filter until the particulates thereon ignite. Using reasonable amounts of power, electric heater systems, however, have had difficulty in achieving a uniform ignition across the front face or inlet end. Since most exhaust systems in vehicles are positioned horizontally, heat from the electric heater which is then situated along a substantially vertical plane, particularly heat which is directed away from the filter, rises and, consequently, on its reradiative or convective return heats the upper portion of the filter much more rapidly than the lower portion. Ignition then occurs first in the region of greater heat. When the flame front passing through the filter is uneven, temperature gradients are severe so that the ceramic may crack and before long break sufficiently to destroy the filter. In an attempt to solve the problem, heating elements have been moved as close as possible to the current face of the ceramic filter. In U.S. patent application Ser. No. 088,055, filed Aug. 21, 1987, a reflective surface is even shown on the side of the heater element opposite the filter. Neither is sufficient to solve the problem using only the power available from the electrical system of a truck. The present invention, however, addresses and solves the problem.